Pulverizer mill



ct. 3, 1939. H, R. COLLINS n 2,174,933

PULVERI ZER MILL Filed Aug. l2. 1936 2 Sheets-Sheet l4 Oct. 3, 1939.y R. CQLUNS 2,174,933

PULVERIZER MILL Filed Aug. 12, 1936 2 Sheets-Sheet 2 lzA 2 M10 Patented Oct. 3, 1939 U'Ni'l' STATES @A TE AN T .OFF' @E PULVERXZER MILL ceased Application August 12, 1936, Serial No. 95,658

18 Claims.

This invention relates to pulverizer mills especially intended for the reduction of non-metallic materials, such as coal, limestones, and Cement clinlzer, by either wet or dry grinding, and in its more specific aspects pertains to mills of the type in which the grinding assembly preferably comprises a plurality of annular series of grinding elements, such as balls, which cooperate with grinding surfaces, one of which is rotatably driven in a horizontal plane to actuate the balls.

Among the objects of the present invention may be noted the provision of a mill of simple .mechanical construction in which reduction of the v,material under treatment is accomplished in a plurality of stages, the structure contemplating an arrangement in which a grinding assembly of the stated character and the driving mechanism therefor are symmetrically disposed about a vertical axis with the driving .mechanism being positioned above the grinding assembly. Further, the driving and grinding vmechanisms arearranged as separate assemblies, independently supportedand self-centering, with the contemplated arrangement permitting assembly, installation, and/or removal of the driving mechanism as a unit without disturbing the grinding assembly, which latter, upon removal of the driving mechanism asaforesaid, is readily accessible and removable so that elements therecfsubjected to wear may be expeditiously replaced when necessary.

rThe present invention further contemplates a pulverizer .mill of the above character in which the weight of certain parts of the grinding assembly is carried by the grinding elements, thus to; apply grinding pressure thereto, and in which the grinding pressure may be further adjusted to the material under treatment in a simple manner and by readily accessible means, with provision being made for the regulation of the degree of neness of the product to satisfy varying requirements.

Other objects will be in part obvious and in part hereinafter pointed out in connection with the accompanying drawings illustrating two embodments of the invention in detail.

In these drawings, wherein like reference characters designate like parts throughout the several figures,

Fig. l is an elevation, partly in section, of a pulverizer mill in accordance with one embodiment of the invention,

Fig. 2 is an elevation, partly in section, of a modified construction in accordance with the invention, and

Fig. 3 is a broken away detail of certain parts thereof.

Referring to the drawings, and more particularly to Fig. 1, reference character I@ designates a generally cylindrical casing constituted by va lower casing section Il, provided with an annular flange I2 which may serve as a bed plate for the apparatus, ,and an upper section I3. Between the casing sections II and I3 is arranged a main bearing support I4, with bolts or equivalent securing means I5 removably securing the bearing support I to facing flanges of the aforesaid casing sections. The upper casing section IS is provided with a horizontal top plate I8 which serves to close the top of the casing i0 as formed by rthe casing section aforesaid.

Arranged on the top plate i8 of casing section i3 and suitably secured thereto is a vertically disposed motor 2i) for the driving assembly, the motor shaft 2| of vwhich extends through the top plate I8 and into a chamber or compartment formed by the casing section I3, its top plate I8, and the main `bearing plate i4.

rhis motor shaft 2| carries a hardened steel pinion-22 which drives oppositely arranged idler gears 23, `24 carried by bearings on suitable vertical shafts 2.5, 2S, respectively, secured in the cover plate I8 of the top casing section. Idler gears 23, 24 mesh with a ring gear 28 carried by a circular supporting plate 30, the latter being secured as by bolts `3| to a flange formed on the upper end of the main driving shaft 32, the upper portion of whichis circular in cross-section.

The main bearing plate I4 is provided with a centralelongated hub 33 disposed about the circular` portion of the shaft 32, the hub being suitably recessed to receive the outer raceways of spaced roller bearings 34, 35, the inner race- .waysbeing carried Yby the -main driving shaft 32. Said bearings are suitably lubricated as by grease injected through a bore (not shown) in the main bearing plate I4. A packing ring 36 operative between the lowerA end of the plate hub 33 and they drivingvshaft `32 serves to exclude entry of dust along the shaft into the bearings and driving g mechanism.

Main bearing plate hub33 .extends upwardly above the top horizontal `surface of the main .bearing platev I4, and hencethere is formed with- 5 in bearing plate i4 and communicating with an oil supply line indicated at 4l.

The bottom plate 3 of the ring gear 28 is provided with one or more apertures as at 42, and threaded'into each of said apertures is a scoop pipe 43 which extends downwardly into the oil reservoir and below the level of the oil contained therein. Upon rotation of the ring gear 28 and its bottom plate, the open-ended scoop pipes 43 pick up oil contained in the reservoir and force it upwardly into a secondary reservoir in the zene of the motor shaft pinion 22, idler gears 23, 2li, and gear surfaces of the ring gear 28, thus to lubricate the same.

Vertical shaft 25 en which idler gear 23 rotates extends through the top plate I8 of casing section I3, and into a two-part housing 45 suitably secured to the casing top plate, the housing 45 ccntaining drive elements for a rotating feed table 46.

The feed table driving mechanism preferably consists of a worm pinion 4l keyed or otherwise secured to the upper end of the vertical shaft 25, the pinion being in driving relation to a gear (not shown) arranged on horizontal shaft 48 suitably journaled in the housing 45, and being provided also with a worm gear surface (not shown) in driving relation to a pinion 5t carried by vertical feed table shaft 5I. Idler shaft 25, the horizontal shaft 48, and the feed table shaft 5l are suitably mounted in bearings as illustrated, the arrangement being such that feed table 4S is driven from the motor shaft 2l through a gear train including pinion 22, idler gear 23, pinions 4l and 55, and the gears on shaft 48.

To lubricate the driving parts contained in housing d5, the top plate I3 of the casing section I3 and the bottom wall of housing 45 are provided with a set of aligned apertures and threaded into said apertures is a scoop pipe 52 (shown in dotted lines), the lower open end of which extends into the cavity formed by ring gear 28 and its bottom plate 30, and below the level of the idler gear 23 rotating on a xed axis therein. Hence, oil forced into the ring gear 2B by scoop pipe 43 is in turn raised by scoop pipe 52 into the housing 45, and functions to lubricate the driving elements therein contained.

The lower end of the main shaft 32 is of square or other angular section and extends through a central opening in the hub portion of a rotary grinding ring 55, which opening is slightly larger than the shaft, whereby the ring is permitted a limited free vertical as well as horizontal movement to maintain alignment to accommodate possible inaccuracies in manufacture or normal wear of the grinding elements. 'Ihe grinding ring 55 is provided with an intermediate portion which is substantially conical and with a horizontal rim portion suitably recessed on its upper side to receive a circumferentially channeled raceway 56 which is welded or otherwise secured to the ring. Supported on the raceway 56 is an upper annular series of grinding balls 51, the top portions of which engage in a circumferentially grooved raceway 53 welded or otherwise secured to the under surface of a non-rotating .grinding ring ii. The upper surface of the non-rotating ring 6 is inclined inwardly and along its periphery is provided with a plurality of spaced spring pockets which open upwardly, with the bottom wall of each pocket being apertured to receive a spring-carrying bolt 62, on which is arranged a compression spring 63 operating between the bottom of the spring pocket and a collar 64 located on the compression bolt by the holding and locking nut assembly designated 65. The said compression bolts at their lower ends are threaded into a shelf ring 66 which seats on an inwardly directed shelf or ledge 6l formed on the inner wall of the casing section Il.

In the above described arrangement the nonrotating grinding or pressure ring 6I), the upper grinding surface 56 of the rotating ring 55, and the annular series of grinding balls 5l constitute an upper grinding stage, in which material to be pulverized is directed to the inner side of the grinding balls by the inclined upper surface of the non-rotating ring 5i! and the conical upper surface of the rotating ring 5E. It will be further seen that the weight of the non-rotating grinding ring 55 of this upper stage is carried by the grinding balls 5l, thus to apply grinding pressure thereto. This grinding pressure may be supplemented by the action of the compression springs limited vertical movement but is held against rotational movement, as by two or more oppositely arranged keys 58 (Fig. 3) formed on the inner wall of the casing section il and engaging in vertical slots or ways 59 formed in the periphery cf the non-rotating ring.

The lower grinding stage is preferably constituted by a circumferentially grooved raceway 1!) welded or otherwise secured to the under rim portion of the rotating ring 55, which engages the top portions of a second or lower series of 4grinding balls l2 supported on a circumferentially grooved raceway i3 welded or otherwise secured to a bottom fixed ring i4. The arrangement is such that both series of balls are vertically aligned and the upper grinding surface of each stage is directly above the lower grinding surface thereof. Hence, grinding pressures are applied in a vertical, downward direction.

The bottom xed ring 'i4 of the lower grinding stage is welded or otherwise secured along its peripheral edge to a cylindrical plate ring 'l5 which is disposed within and spaced from the casing section H, and has its upper end welded or otherwise secured to the fixed ring 66. Thus, the fixed ring 14 of the lower grinding stage is supported from the shelf ring 6l, and in turn supports the upper and lower series of grinding balls, upper grinding ring 5i), and rotary `grinding ring 55.

Fixed shelf ring 5E is removably secured on its supporting shelf as by holding bolts 16 which extend through thickened portions of the casing section Il and bear downwardly on the ring, thus to securely maintain it seated on the shelf 6l. When it is desired to remove the ring 66 and the entire grinding assembly carried thereby as in the foregoing, holding bolts 'I6 are unthreaded, and raising bolts Tl, operating through the casing wall against the under face of the ring, function to unseat the ring 56 from the shelf 6l, whereupon the said ring and parts carried thereby may be raised for removal from the casing.

The material is discharged from the upper grinding stage through an inclined screen 'H8 carried by the rotary ring 55 and arranged adjacent the outer side of the grinding balls 5l. The material thence drops downwardly to the annular space inwardly of the cylindrical plate ring 'I5 and is directed to the outer side of the lower grinding 75 balls v1.2. Following suitable treatment in the lower stage, the pulverized material is discharged from the inner side of the balls 12, under the control of an adjustable height dam 'i9 and screen 8B secured in suitable manner to the inner edge of the fixed grinding ring 14. The material is thereupon picked up by air entering the casing through an air inlet opening 8l in the lower portion of casing section ll toa fan 82. After passing through a suitable grating'83, the pulverized material enters the fan, by which it is'forwarded as desired.

The fan 82 is preferably driven -by a shaft-'84 which passes upwardly Ythrough a'bore 85 in the main driving shaft 32 and is secured tothe hub of the driving pinion 22. The lower end of the shaft 84 is supported for rotation in a bearing (not shown) formed in the lower fan section. lIt will thus be seen that the motor drives the fan 82 at full load speed, whereas the grinding balls of both grinding stages are driven circumferentially of the mill at a speed which preferably corresponds to the gravity fall ofthe material, through the speed reducing effect ofthe driving mechanism as aforesaid.

It will be understood that a'slurry pump will be substituted for the fan 32 when wet grinding is contemplated.

Considering now themodification illustrated in Fig. 2, the driving mechanism is preferably .substantially the same as above described. The rotating grinding ring'55 is driven from the driving shaft 32 and the grinding assembly comprises an upper stage consisting of an upper series of grinding balls 51 operative between circumferentially channeled raceways 56 and 58 carried, respectively, on the upper surface of the rotating ring and the under surface of the non-rotating pressure ring 6i), all as aforesaid; and a lower stage consisting of a lower annular series of grinding balls 'l2 operating between raceways one of which, 10, is carried on the under side of the rotating grinding ring t5 and the other, i3, is seated on the upper side of a fixed grinding ring 14A, the relatively outer edge of which is formed with an overhanging flange 'MB which seats 0n an inwardly directed shelf or ledge I2A formed on the casing bed plate I2.

Spring-carrying bolts 62A extend through the non-rotating pressure ring 66 as previously described, and their lower ends extend through aligned apertures formed in the overhanging fiange "MB of the bottom grinding ring and the inward flange I2A of the bed plate l2. Bolts 62A are each provided with an expanded collar 62B arranged to bear against the top face of overhanging flange '14B so that, by tightening up the lower nuts 62C, the collars 62B are drawn tight against the overhanging flange and serve not only to removably secure the fixed bottom ring 14A to the casing, but also to anchor the lower ends of the compression bolts 62A.

To remove the lower fixed ring 14A and the entire grinding assembly carried thereby, certain of "the expanded collars 62B engage beneath forked clamps @2D bolted or otherwise secured to the overhanging iiange of the said lower ring. The arrangement is such that, upon unthreading the lower tightening nuts 62C, the forked clamps secure the lower fixed ring 74A to the expanded collars whereby said ring and parts carried thereby may be lifted bodily by the compression bolts 62A.

The path of material flow in both the Figs. 1 and 2 constructions is similar, the material entering' the casing inregulated quantity through spout 8D under the control of the work feed table 46 previously described. The inclined surfaces of the top pressure ring Gil and the rotating intermediate ring 55 thereupon direct the material to the inner side of the grinding balls cf the upper grinding stage, being discharged therefrom through -the screen 18, and falling to the outer side of the grinding balls l2 of the lower grinding stage, from which it is controllably discharged as by the adjustable height dam 'I9 and associated screen ilas previously described.

In the Fig. 2 construction, an exhaust fan 82A is suitably secured to the under side of the casing or bed plate thereof, the air entering thereto through the inlet openings BIA at the bottom of the casing, with the inlet air in the present modi- ,fication being drawn directly through the grinding calls of the lower stage and screen, thus to assist in the discharge of the finished product.

In puiverizer mills according to the foregoing, the symmetrically disposed driving and grinding mechanisms are separate assemblies. Thus, by removal of the holding bolts l 5, top casing section i3 may be lifted, thus to permit access to the driving mechanism supported by the main bearing piate l, and, further, by raising the main bearing plate lei, the complete driving assembly may be removed from the casing to permit access to the grinding assembly or to permit removal of the latter for the purpose of replacing worn parts.

Further, by the unique grinding assembly of the present invention, grinding pressures can be adjusted to the material under treatment, with the dams 19 on the outlet side of the lower annular series of grinding balls acting to control the degree of iineness of the material passing through screens 80. Accordingly, particle sizes can be closely controlled, and the resulting product will be substantially uniform in size. The disposition of compression bolts and springs within the casing and outwardly of the grinding balls further aids in the free flow of the material through the mill, and represents simplified constructional and design features.

As many changes could be made in carrying out the above construction without departing from the scope of the invention, it is maintained that all matter contained in the above descrip tion or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

l. A pulverizer mill comprising, in combination, a casing, upper and lower non-rotating grinding rings, an intermediate rotating ring, 5L,

upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, a shelf ring removably supported within said casing and disposed about the grinding elements, means carried wholly by said shelf ring and operative to exert grinding pressure on said grinding elements, means suspending the lowei` grinding ring from the shelf ring, and means for driving the rotating ring.

A pulverizer mill comprising, in combination, a casing, upper and lower non-rotating grinding rings, an intermediate rotating ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, a shelf ring removably supported within the casing and arranged about'the grinding elements, spring'means carried wholly by the shelf ring and operative through the upper ring to exert grinding pressure on the grinding elements, means suspending the lower grinding ring from the shelf ring, and means for driving the rotating ring.

3. A pulverizer mill comprising, in combination, casing, upper and lower non-rotating grinding rings, an intermediate rotating ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, a shelf ring removably supported within the casing and arranged about the grinding elements, spring means for exerting grinding pressure on the grinding elements, said means comprising compression bolts anchored in said shelf ring, compression springs arranged on said bolts and operative against the upper ring, means suspending the lower ring from the shelf ring, and means for driving the rotating ring.

4. A pulverizer mill comprising, in combination, a casing, upper and lower non-rotating grinding rings, an intermediate rotating ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, a shelf ring removably supported within said casing and disposed about the grinding elements, means carried wholly by said shelf ring and operativ-e to exert grinding pressure on said grinding elements, a downwardly extending plate ring carried by the shelf ring, the lower ring being secured to the plate ring, and means for driving the rotating ring.

5. A pulverizer mill comprising, in combination, a casing, upper and lower non-rotating grinding rings, an intermediate rotating ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, a shelf ring removably supported within the casing and arranged about the grinding elements, spring means carried wholly by the shelf ring and operative through the upper ring to exert grinding pressure on the grinding elements, a downwardly extending plate ring carried by the shelf ring, the lower ring being secured to the plate ring, and means for driving the rotating ring.

6. A pulverizer mill comprising, in combination, a casing, upper and lower non-rotating grinding rings, an intermediate rotating ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, a shelf ring removably supported within said casing and arranged about the grinding elements, spring means for exerting grinding pressure on the grinding elements, said means comprising bolts anchored in the shelf ring, and compression springs arranged on said bolts and operative against the ring, a downwardly extending plate ring carried by the shelf, the lower ring being secured to the plate ring, and means for driving the rotating ring 'l'. A pulverizer mill Icomprising, in combination, a casing, upper and lower non-rotating grinding rings, an intermediate rotating ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, a shelf ring removably supported within said casing and disposed about the grinding elements, means carried by said shelf ring and operative to exert grinding pressure on said grinding elements, a downwardly extending plate ring carried by the shelf ring, the lower ring being secured to the plate ring inwardly thereof, the plate ring being spaced from the casing whereby to provide an annular passage, and means for driving the rotating ring.

8. A pulverizer mill comprising, in combination a casing, a grinding assembly including upper and lower non-rotating grinding rings, an intermediate rotating ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, the casing including a bed plate, cooperating means on the bed plate and the lower ring for supporting the lower ring, means arranged wholly within the casing and outwardly of the grinding elements for exerting grinding pressure thereon, said means also providing a connection between the non-rotating rings whereby the grinding assembly may be raised bodily from the casing, and means for driving said rotating ring.

9. A pulverizer mill comprising, in combination, a casing, a grinding assembly including upper and lower non-rotating grinding rings, an intermediate rotating ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, the casing including a bed plate having an extension projecting into the casing, the lower ring being supported on said extension, means arranged vvholly Within the casing and outwardly of the grinding elements for exerting grinding pressure thereon, said means also providing a connection between the non-rotating rings whereby the grinding assembly may be raised bodily from the casing, and means for driving the rotating ring.

10. A pulverizer mill comprising, in combination, a casing, a grinding assembly including upper and lower non-rotating grinding rings, an intermediate rotating ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, the casing including a bed plate having a flange extending into the casing, the lower ring having an overhanging liange seating on the bed plate flange, means arranged wholly within the casing and outwardly of the grinding elements for exerting grinding pressure thereon, said means also providing a connection between the non-rotating rings whereby the grinding assembly may be raised bodily from the casing, and means for driving the rotating ring.

ll. A pulverizer mill comprising, in combination, a casing, upper and lower non-rotating grinding rings, an intermediate rotating ing, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings', the casing including a bed plate having a iiange extending inwardly of the casing, bolts removably anchored in said flange and extending upwardly through said upper ring and outwardly of the grinding elements, said bolts providing a connection between the non-rotating rings, compression springs arranged on the bolts and operative against the upper grinding ring, and means for driving the rotating ring.

12. A pulverizer mill comprising, in combination, a casing, upper and lower non-rotating grinding rings, an intermediate rotating ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, said casing including a bed plate having a flange extending into the casing, the lower ring having an overhanging flange seated on the bed plate flange, bolts secured to said flanges and extending through the upper ring and outwardly of the grinding elements, said bolts providing a connection between the nonrotating rings, compression springs carried by said bolts and operative against the upper ring, and means for driving the rotating ring.

13. A pulverizer mill comprising, in combination, a casing including a bed plate provided with a flange extending into the casing, and a grinding assembly carried by said flange, said grinding assembly including a lower ring seating on said flange, a rotating ring, and an annular series of grinding elements arranged between and engaging against the lower and rotating rings, bolts secured to the lower ring and extending upwardly within the casing, and compression springs carried by said bolts operative to exert grinding pressure on the grinding elements, the outer di ameter of the lower ring being less than the inner diameter of the casing whereby the grinding assembly may be lifted from said bed plate flange through the medium of the compression bolts.

14. A pulverizer mill comprising, in combination, a vertically arranged casing having an inwardly directed shelf, a grinding assembly disposed within the casing and including upper and lower non-rotating grinding rings, an intermediate rotating grinding ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, and means wholly contained within the casing for exerting yielding grinding pressure on the grinding elements and for securing the rings and elements together as a self-contained assembly-unit, means including the shelf providing a support within the casing for the lower non-rotating ring and hence for the grinding assembly, from which support the grinding assembly may be raised as a unit, and means for driving said rotating ring.

15. A pulverizer mill comprising, in combination, a vertically arranged casing having an inwardly directed shelf, a grinding assembly disposed withinthe casing and including upper and lower non-rotating grinding rings, an intermediate rotating grinding ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rin-gs, and means wholly contained within the casing for exerting yielding grinding pressure on the grinding elements and for securing the rings and elements together as a self-contained assembly unit, means including the shelf providing a support within the casing for the lower nonrotating grinding ring and hence for the grinding assembly, releasable means for securing the lower non-rotating grinding ring to the shelf, and means for driving said rotating grinding ring.

16. A pulverizer mill comprising, in combination, a vertically arranged casing having an inwardly directed shelf, a grinding assembly including upper and lower non-rotating grinding rings, of which the lower ring is provided with a peripheral extension, an intermediate rotating grinding ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, and means wholly contained within the casing for exerting yielding grinding pressure on the grinding elements and for securing the rings and elements together as an assembly unit, means including the shelf providing a support within the casing for the lower non-rotating grinding ring and hence for the grinding assembly, and from which support the grinding assembly may be raised as a unit, said pressure-exerting and securing means including vertically arranged spring-carrying bolts, the lower ends of which are fixedly secured to said peripheral extension of the lower nonrotating grinding ring and the upper portions of which extend through the peripheral portion of the upper non-rotating grinding ring, and means for driving said rotating grinding ring.

17. A pulverizer mill comprising, in combination, a vertically arranged casing having an inwardly directed shelf, a grinding assembly including upper and lower non-rotating grinding rings, of which the lower ring is provided with a peripheral extension, anintermediate rotating grinding ring, upper and lower annular series of grinding elements disposed between and engaging respectively the upper and intermediate rings and the intermediate and lower rings, and means for exerting yielding grinding pressure on the grinding elements and for securing the rings and elements together as an assembly unit, means including the shelf providing a support within the casing for the lower non-rotating grinding ring and hence for the grinding assembly, and from which support the grinding assembly may be raised as a unit releasable means for securing the lower non-rotating grinding ring to the support, said pressure-exerting means including vertically arranged spring-carrying bolts, the lower ends of which are xedly secured to said peripheral extension of the lower non-rotating grinding ring and the upper portions of which extend through the peripheral portion of the upper non-rotating grinding ring, and springs carried by said bolts operative against the upper side of the upper non-rotating grinding ring, means for directing material to be pulverized to the inner side of the grinding elements of the upper Series and for discharging the pulverized material from the inner side of the grinding elements of the lower series, and means for driving the rotating grinding ring.

18. A pulverizer mill comprising, in combination, a casing, a shelf ring arranged within the casing, means in the casing providing a seat for the shelf ring, from which the ring may be raised, a grinding assembly disposed within the casing and including a non-rotating ring arranged at a substantially lower level than said shelf ring, an annular series of grinding elements supported thereon, an upper rotating ring supported on said grinding elements and means carried by said shelf ring for exerting yielding grinding pressure on said grinding elements and for securing said rings and elements together as a self -contained assembly unit, and means within the casing and extending between the shelf and lower rings for supporting the lower ring from said shelf ring.

HARRY R. COLLINS. 

